Studies will be directed toward the elucidation of humoral immune mechanisms that may be operative 1) in experimental and human syphilis and 2) following vaccination of rabbits with inactivated Treponema pallidum, utilizing the methods of passive protection and neutralization. Studies will also be directed toward the elucidation of cell-mediated immune mechanisms that may be operative in the experimental and human disease and following vaccination of rabbits. Additionally, an attempt will be made to develop an experimental congenital and neonatal syphilis model in the rabbit and to elucidate possible humoral vs. cellular immune mechanisms which may influence resistance and pathogenesis.